1. Field of the Invention
This invention relates to a method and electrical circuitry for low noise transmission of electrical signals, and more particularly to a compander signal transmission method and circuitry therefore.
The subject matter of the present invention was disclosed in Disclosure Document entitled "Noise Reduction System for Optical Motion Picture Soundtracks", No. 050581, filed July 2nd, 1976.
2. Description of the Prior Art
Compander transmission is used to transmit information signals having relatively large variances in signal level over a limited amplitude transmission channel which is incapable of accurately handling the full amplitude range of the information signal. According to this method, the signal is compressively encoded for transmission so that the relative gain of its higher level portions is reduced with respect to its lower level portions, whereby the signal after encoding occupies a reduced amplitude range. At the receiving end the transmitted signal is decoded by increasing the relative gain of the higher level signals in a fashion complementary to the encoding process.
The first systems of this type were employed to improve the signal-to-noise quality of telephone communications One form of compander used in telephone systems and more recently in other audio systems is the syllabic compander. In this type of compander, illustrated in FIG. 1, the signal envelope level is used to control the signal gain in transmission and playback. An information signal at input terminal 2 is transmitted to a variable gain amplifier 4, and also to a level sensing circuit 6 which senses the amplitude of the signal envelope. The output of the level sensing circuit is conditioned by a non-linear circuit 8, which controls the gain of amplifier 4 so as to amplify lower level signals more than higher level signals. The encoded output of amplifier 4 is transmitted over a transmission channel 10 to a decoding section comprising a level sensing circuit 12, non-linear circuit 14, and variable gain amplifier 16 connected similarly to the encoding section. The decoder non-linear circuit 14 is complementary to encoder non-linear circuit 8, causing decoder amplifier 16 to expand the transmitted signal and thereby reproduce the original information signal at its output.
A significant problem encountered with the compander circuit described thus far is that the transient response time of the level sensing circuits must be slow so as to restrict the bandwidth of the gain control signal compared to that of the encoded signal. This slow transient response time manifests itself as a degradation of the total system transient response, which phenomenon is illustrated in FIGS. 2a-2c. FIG. 2a illustrates an input information signal characterized by an initial low level or quiescent state 18, followed by a high level signal burst 20, and finally a return to a low level state 22. FIG. 2b illustrates a typical level sensing response to such an input signal. The gain control signal produced by the level sensing circuit is relatively high during the initial period 24. The transient response of the gain control signal at the appearance of the large amplitude signal burst 20 is indicated by the sloped portion 26 of the curve. As can be seen, the signal does not reach its minimal level until point A, which depending on the type of level sensing used might typically occur about five milliseconds after the information signal burst is first detected. The gain control signal remains at the reduced level until point B, corresponding to the end of the information signal burst, after which it gradually increases over another transient response period, indicated by sloped portion 28, back to the original quiescent level.
The transmitted signal, illustrated in FIG. 2c, is obtained by multiplying the information and gain control signals together. During the transient response period 26, however, the gain control multiplier has not yet reached its final minimum level and is too high for the corresponding information signal. The resulting transmitted signal level is also too high during this period and, if it exceeds the dynamic range of the transmission channel, the peaks of the transmitted signal which fall outside that range will be chopped away. This situation is illustrated by the chopped signal 30 in FIG. 2c. At the receiving end of the system the transient is further distorted because the level sensing circuit at that end can increase the gain of its variable gain amplifier only slowly, resulting in a transient output that is further reduced in level. The ultimate result is that the transient portion of the restored signal after transmission is not only attenuated but also distorted. The more severe the compression used in the compander system, the more severe is the attenuation and distortion of transients. Further, the effectiveness of the compander in reducing noise is mitigated by the relatively slow decay of the gain control signal at 28. The result is that in decoding, the gain is maintained relatively high for a considerable time after the end of the transient burst, making channel noise more objectionable.
The distortion problem has been particularly annoying in past attempts to optically record multiple audio tracks on cinemagraphic film. The same film area is generally allocated to audio regardless of how many tracks are used. This area must be divided up between each of the audio tracks, with a further portion allocated to dead space separating the tracks. While the film area is generally adequate for single track recordings, attempts to simultaneously record all four tracks of a quadraphonic system have been characterized by recorded signals which frequently exceed the dimensional limits of their respective tracks. While this problem might be attacked by increasing the compression of the encoded signals to reduce their amplitude range, such an approach also increases signal noise due to the reduced dimensions of the recorded signal relative to film graininess, dirt, and dust.